The invention relates to a mixer arrangement comprising a variable gain amplifier stage including a first field effect transistor T.sub.1 whose first principal terminal referred to as drain is biased via a load T.sub.3 by a DC supply source V.sub.DD and whose control terminal referred to as gate receives a first RF signal to be mixed with a second signal OL applied to the first field effect transistor T.sub.1 through a variable resistor T.sub.2, such that the output signal IF of the amplifier stage available at the first principal drain terminal of the first transistor T.sub.1 is formed from the mixture of the first and the second signal.
This arrangement is used for processing, for example, hyperfrequency signals in the field of receiving television transmission signals.
Such a mixer arrangement is known in the prior art from the publication by Rory Van Tuyl in "1980 IEEE Solid State Circuits Conference" in the issue of "ISSCC 80, Thursday, Feb. 14, Session X=Microwave Circuits, pages 118, 119" entitled "A monolithic GaAS FET RF Signal Generation Chip".
This publication particularly describes a mixer arrangement comprising an amplifier stage comprising a first field effect transistor constituting the actual amplifier section. The control or gate electrode of the first transistor receives a first signal at a first frequency which is mixed by a second signal at a second frequency. This second signal is applied to the source of the first transistor by means of a second field effect transistor used as a variable resistor. The drain of the first transistor, referred to as amplifier is biased from a DC supply through a load, and its source is biased with respect to ground by a third transistor being arranged as a current source, resulting in this amplifier stage functioning as a negative feedback amplifier. The mixture of the first and the second signal is available at the drain electrode of the first transistor, and constitutes the output signal of the variable gain amplifier. The third transistor, utilized as a current source, has a high impedance in order to interfere as little as possible in the mixer function. The second transistor, used as a variable resistor receives the signal from a local oscillator at its gate electrode and is given a condition in which its drain-source voltage is equal to zero.
The document cited above particularly describes a double and balanced arrangement of this mixer, permitting improvement of the isolation between the gates; that is to say, obtaining a satisfactory isolation between the signal from the local oscillator, the input signal of the mixer and the output signal.
The mixer circuit described in the document cited above has two serious drawbacks. The first drawback is that the noise factor of this circuit is very high. This is due to the presence of noisy elements applied to the source of the first transistor referred to as amplifier. A noisy element is, inter alia, the transistor used as a current source. The second drawback is that the satisfactory biasing conditions of the amplifier transistor are difficult to obtain.